A prior art technique of this kind is disclosed in Japanese Patent Laid-Open No. 93,939/1980 entitled "Heat Exchanger in External Combustion Engine." In this prior art technique, heater tubes for a Stirling engine are disposed inside the heating chamber of a heater. The space in which the heater tubes are located is filled with inert gas. The heater tubes are heated by a high-temperature heat source located outside the heating chamber via the inert gas. This technique is intended for homogeneous distribution within the heater tubes and also for protection of the tubes against heat.
This prior art technique has certain problems. For example, when the amount of heat supplied from the high-temperature heat source varies, the amount of heat supplied from the heater head of the Stirling engine and used in operating the cycle changes. This heat change varies the shaft horsepower of the engine, and, as a result, a stable output cannot be obtained.